Faking the speed/vector pass
Vector Pass: Motion vectors for the vector blur node. The four components consist of 2D vectors giving the motion towards the next and previous frame position in pixel space. [ref: https://docs.blender.org/manual/en/dev/render/cycles/settings/scene/render_layers/passes.html ]

The VectorPass stores these 2 vectors using an RGBA only the node editor renders the socket as being a vector. The R+G channels are used to store the motion towards the previous frame. and the B+A channels are used to store the motion towards the next frame.

Your goal of influencing this in a material is possible by using a Transparency Shader and Emision Shader and mix those together.